edit: Oh, never mind, yellow will be hot even if the car is off. Duh.

Right. Even if you were to use the Orange (ignition) wire instead of Yellow, that would still be a bad thing because the amp needs to be turned off in a lot of situations where the ignition would otherwise be on. If you were to wire it to the ignition wire, then you would get a lot of speaker-damaging "pops" at times like engine starting and shutting off.

The amp remote line has a very specific set of conditions under which it turns on and off, and those have been carefully planned. Any way of wiring around that is a recipie for trouble.